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14 Classical Mechanics and the Old Quantum Theory
(a)
t 1 :
t 2 :
t 3 :
t 4 :
(b)
t 1 :
t 2 :
t 3 :
t 4 :
A traveling wave:
A standing wave:
Figure 14.5 Traveling and Standing Waves. (a) A traveling wave at times t 1 < t 2 < t 3 <
t 4 . (b) A standing wave at times t 1 < t 2 < t 3 < t 4 .
Scattering centers
Wave trough
Wave crest
Wave front
Wave crests
4l
3l
2l
l
l
Figure 14.6 The Diffraction of a Wave by a Row of Scattering Centers. The direction
of the diffracted wave depends on the wavelength.
interfere to produce a diffracted wave with straight crests, which can travel in a direction
different from that of the incident wave. Light waves and other waves exhibit the same
phenomenon.
A plane wave is a traveling wave in three dimensions in which a crest of the wave
occupies a plane at right angles to the direction of propagation. Figure 14.6 illustrates
diffraction of a plane wave by a set of equally spaced scattering centers. The broken lines
in Figure 14.6 represent the crests of a plane wave moving from left to right. The arcs
represent the crests of diffracted waves moving outward from the scattering centers.
At a large distance from the scattering centers these crests interfere to produce a plane
wave moving in a different direction from the original direction of propagation. The